Fundamentally new technologies are required to continue increasing device integration density and speed. Conventional metal-oxide semiconductor-field-effect transistors soon will reach fundamental density and speed limits as a result of quantum mechanical tunneling. In order to scale electronic device sizes down to nanometer dimensions, systems containing increasingly fewer numbers of particles must be considered.
The ultimate limit is a system in which the transfer of a single charge quanta corresponds to information transfer or some type of logic operation. Such single-electron systems are presently the focus of intense research activity. See, for example, Single Charge Tunneling, Coulomb Blockade Phenomena in Nanostructure, edited by H. Grabert and M. H. Devoret, NATO ASI. Series B: Physics Vol. 294 (1992). These systems have potential application to nanoelectronic circuits that have integration densities far exceeding those of present day semiconductor technology. See, Quantum Transport in Ultrasmall Devices, edited by D. K. Ferry, H. L. Grubin, C. Jacoboni, and A. Jauho, NATO ASI Series B: Physics Vol. 342 (1995).
Single-electron transistors based on the concept of Coulomb blockade are one proposed technology for realizing ultra-dense circuits. K. K. Likharev, Single-electron Transistors: Electrostatic Analogs of the DC SQUIDS,” IEEE Trans. Magn. 23:1142 (1987); and IBM J. Res. Dev. 32:144 (1988). Coulomb blockade is the suppression of single-electron tunneling into metallic or semiconductor islands. In order to achieve Coulomb blockade, the charging energy of an island must greatly exceed the thermal energy. To reduce quantum fluctuations the tunneling resistance to the island should be greater than the resistance quantum h/e2. Coulomb blockade itself may be the basis of conventional logic elements, such as inverters. Id.
Equally promising is the fact that the Coulomb blockade effect can be used to pump charges one-by-one through a chain of dots to realize a frequency-controlled current source in which the current is exactly equal to I=ef, where f is the clocking frequency. See, L. J. Geerligs et al., Frequency-locked Turnstile Device for Single-electrons, Phys. Rev. Lett., 64:2691 (1990); and H. Pothier et al., Single-Electron Pump Based on Charging Effects, Europhys. Lett. 17:249 (1992). Such turnstile devices are of fundamental interest as highly accurate current standards.
The clocking of charge through an array is also one model of information storage. It is possible that computation may be based on switching of currents rather than charge which, due to the extreme accuracy of single-electron current sources, may be more robust towards unwanted fluctuations than single-electron transistor-based circuits.
One of the most promising technologies for realizing terabyte memories is founded on the principle of the Coulomb blockade. Yano et al. have demonstrated room temperature operation of single-electron devices based on silicon nanocrystals embedded in SiO2. K. Yano et al., Room-Temperature Single-electron Memory, IEEE Trans. Electron. Devices, 41:1628 (1994); and K. Yano et al., Transport Characteristics of Polycrystalline-Silicon Wire Influenced by Single-electron Charging at Room Temperature, Appl. Phys. Lett., 67:828 (1995). Recently, a fully integrated 8×8 memory array using this technology has been reported. K. Yano et al., Single-Electron-Memory Integrated Circuit for Giga-to-Tera Bit Storage, IEEE International Solid State Circuits Conference, p. 266-267 (1996).
Microelectronic devices based on the principle of Coulomb blockade have been proposed as a new approach to realizing electronic circuits or memory densities that go beyond the predicted scaling limit for present day semiconductor technology. While the operation of Coulomb blockade devices has been demonstrated, most operate only at greatly reduced temperatures and require sophisticated nanofabrication procedures. The size scales necessary for Coulomb blockade effects at such relatively elevated temperatures of about room temperature impose limits on the number, uniformity and connectivity of quantum dots. As a result, alternative methodologies of nanofabrication need to be investigated and developed.